1. Field of the Invention
An object of the present invention is a device for the conversion of an image. The conversion made is that of an image, transmitted by electromagnetic radiation, into an electronic image. In a preferred example, the electromagnetic radiation is an X-radiation. However, it may be a radiation in the visual domain. The field of the invention is chiefly that of radiological image intensifiers or RII. It may also be that of light image intensifiers or LII. Intensifiers of this kind, in addition to conversion, carry out an amplification of the image signal.
2. Description of the Prior Art
FIG. 1 shows an image intensifier device. For example, in the medical field, an X-ray tube 1 irradiates the body 2 of a patient. An anti-scattering grid 3 eliminates the rays that are not radial from the X-radiation going through the body 2. In an electron tube 4, a photocathode 5 delivers electrons focused on a target 6. The photocathode is excited by the radiation to be converted and locally, at each place where it is excited, produces an electron radiation whose intensity is proportional to the intensity of the incident electromagnetic radiation. In the field of radiology, the photocathode is associated with a scintillator that converts the X-rays, which have very short wavelengths, into electromagnetic rays that have a greater wavelength and are capable of exciting the photocathode 5. The electrons are attracted towards the target by the presence of an anode. The electrons are furthermore subjected to deflections imposed by an electrical focusing field. The electrical field is induced by a set of electrodes 7 taken to appropriate levels.
When they are liberated from the photocathode 5, the speed of the electrons is very low. The speed of the electrons, combined with their charge, constitutes an electrical current. The electrons are then unfortunately, according to Lenz's law, subjected to parasitic deflections dictated by all the existing magnetic fields on their paths. The most widely known deleterious magnetic field is that resulting from the earth's magnetic field.
The focusing device itself contributes known deformations to the image. The correction of these deformations has already been considered in the prior art. The best known deformation is the pincushion distortion. It is due to the spherical nature of the input face of the tube 4. It is possible, with correction electrodes as well as with target-reading electronic devices, to correct it accordingly.
The deformation dictated by parasitic magnetic influences is an S deformation. It has a twofold effect. Firstly, with respect to a component, transversal to the focusing axis, of the deleterious magnetic field, it results in a substantially homogeneous (first order) translation of all the points, or pixels, of the image on the target. Secondly, with respect to the axial component of the deleterious magnetic field, it gets combined with the component, transversal to the focusing axis, of the speed of the electrons. It leads to a differential rotation of the image around the focusing axis. The amplitude of this rotation depends on the transversal component of speed and the non-homogeneous attenuation of the magnetic shielding of the tube. It is known that, under these conditions, the rotational distortion of the pixels of the image obtained is all the greater as the distance of these pixels from the center of the image is small.
Compensation for these latter distortions has been envisaged in the prior art. A first approach has consisted in providing an envelope 8 of the image intensifier tube with a layer of magnetic material to channel the disturbing magnetic fields in this layer. The best known magnetic material used is .mu. metal. This .mu. metal is an alloy of nickel and iron that concentrates the field lines. It is thus possible to provide the input 9 of the tube with a layer of magnetic material of this kind, but with a very small thickness, in order to obtain better protection.
In order to try and eliminate the most harmful effects of the axial component of the terrestrial magnetic field, it has even been planned to place a coil 10 very close to the input of the tube 4 producing an axial magnetic field but with a value opposite the value of the axial component of the terrestrial magnetic field. Whereas, without correction, the rotations of the pixels under the effect of the distortion may be about 10 mm, with these compensation means, they may be reduced by half. However, in the case of high-resolution images, where the size of a pixel is about 200 to 300 micrometers, a distortion of this kind is still equivalent to a distance of 15 to 25 pixels. This is far too much for certain applications.
The target 6 consists of a layer of luminophors that emit light under the excitation of electron rays, by cathodoluminescence effect. The image formed on the target 6 is then read by different devices. For example, it may be read by a cinema camera 11. In this case, a succession of images produced on the target 6 is recorded. The image may also be read, if it is unique, by a photographic apparatus 12. In a preferred solution of the invention, the image is read by a television camera 13. In particular, the camera 13 digitizes the image.
Within the framework of this preferred use, there is a known way of correcting the distortions resulting from the parasitic influences of the magnetic field by using a digital image processor 14 linked with the camera 13. The corrected image or the unprocessed image is presented on a monitor 15. The principle of the correction consists in reading an image of a test pattern placed in the path of the electromagnetic radiation, for example; in the input plane 9 of the RII. The test pattern is known by construction and constitutes the reference of the non-distorted image. With the series of elements 4, 13, 15, the image of the test pattern obtained reveals the distortions due to the magnetic field in the conditions of acquisition. The processor 14 then compares the perfect image of the test pattern with the revealed image of the test pattern. This comparison gives a piece of information on the distortion undergone by the image and imposed by the series of elements 4, 13, 15. From this distortion information, it is possible to compute a reverse distortion function. The reverse distortion function is then applied to the digital image of the patient's body 2 delivered by the camera 13 in order to correct it.
This technique is implemented especially in tomodensitometers. Indeed, for these instruments, on the one hand, a precision of one-tenth of a pixel is sought. On the other hand, fortunately for these machines, the orientations of the tube 4 in space with respect to the earth's magnetic field can easily be identified. Indeed, machines of this kind have an axis of rotation, with the tube 4 having to occupy predetermined radial positions around this axis of rotation. It is therefore possible, for each rotation of the tube 4 about this axis of rotation, to detect a reverse distortion function and index the correction of the images delivered by the tomodensitometer as a function of this angle of orientation during the acquisition.
However, a technique of this kind cannot be used in an apparatus for which the position of the RII is not identified, especially in the context of radiology instruments comprising an arm incurvated in the form of an arc of a circle on which the tube 4 shifts rotationally. These instruments are commonly called C-arms. Indeed, this incurvated arm is itself fixed to a shaft that enables the rotation of this arm around a second axis of rotation perpendicular to the axis of rotation of the tube 4 along the incurvated arm. Furthermore, the arm is itself mounted on a rotational pivot. Consequently, the tube 4 has three degrees of freedom in rotation. For each of these degrees, the tube 4, depending on need, may occupy any place. Consequently, the map of the reverse distortion functions to be detected is infinite. In practice, this approach cannot be used for instruments of this type.
It is an object of the invention to overcome this problem by noting that the useful images are not permanently acquired by the tube 4. In the invention, an image of the test pattern is then acquired almost in real time, during, before or after the acquisition of each image of the body. To achieve this more easily, the invention comprises means mounted fixedly in the tube 4 to constitute an image of the test pattern in real time. In one example, this can be achieved in two ways. Firstly, a periodic pattern, or a grid, that alters all the images in a known way is incorporated into the input of the tube. The alteration occurs geographically at places whose position on a theoretical image (without distortion) is known beforehand. The effects of these alterations in the real image are registered and compared with the theoretical image and a correction to be made to the useful image of the body is deduced therefrom. In another mode, the alteration is not permanent. It may or may not be provided in real time to the useful image. For example, the photocathode is illuminated intermittently with an auxiliary light radiation producing therein traces that represent the grid. Or else, the image of the test pattern is scrambled in the useful image during the acquisition of the useful image, and then the image of the test pattern is not scrambled during the acquisition of the image of the test pattern. According to the invention, in these cases of definitive or non-definitive alteration, it is possible to obtain an alternating reading of the useful image and of the image of the test pattern. In these two cases, it will be seen that it is also possible to carry out a simultaneous reading of the two images.